In the state of the art, the CdTe solar cell has the following structure, wherein the layers are arranged in the mentioned sequence: a transparent substrate, for instance made of glass; a transparent conducting oxide layer (TCO) formed as front contact; a layer of cadmium sulfide (CdS); a layer of cadmium telluride (CdTe); and a metal layer to collect the charge carriers.
In producing the solar cells in the so called superstrate configuration, the transparent substrate forms the base on which the subsequent layers are deposited one after another. In the production process of solar cells in the so called substrate configuration, the steps of forming the individual layers are performed basically in a reverse order. That is, the process starts with forming a back contact on a suitable substrate or with providing the back contact in form of a substrate, for instance as a flexible metal foil such as molybdenum, on which the subsequent layers are deposited one after another.
The transparent conducting oxide layer (TCO) advantageously comprises a high resistive transparent buffer layer. The use of the high resistive buffer layer is very important, since this buffer layer helps to minimize the shunting effect in the solar cell. Thus, this layer increases performance, reliability and reproducibility of the produced solar cells.
It has been proved that cadmium stannate (CTO) is an optimal conducting material for CdTe solar cells and zinc stannate (ZTO) is the suitable high resistive buffer layer partner.
Many research work from the literature show that the formation of stoichiometric and polycrystalline CTO film is a difficult issue. In general, the CTO layer needs very high temperature in order to obtain crystalline phases. Moreover according to the state of art, the CTO layer has cadmium deficiency independent of deposition methods. In case of CTO layer preparation by sputtering, the formed film is amorphous in general and has to be annealed in a temperature range up to 600° C. for forming a crystalline film. Therefore, the material to be used as substrate is very limited. In particular, cost efficient soda lime glasses cannot be used as substrate, since it is not resistant to elevated temperatures.
From “CdS/CdTe thin-film solar cell with a zinc stannate buffer layer” (Wu et al., Photovoltaic program review meeting, Denver/Colo., Sep. 8-11, 1998) it is known that zinc stannate films (Zn2SnO4 or ZTO) are suited as a buffer layer, since they have a high bandgap, high transmittance, and low surface roughness.
Another method for forming stannate thin films is the spray pyrolysis technique. For instance, “Preparation of cadmium stannate films by spray pyrolysis technique” (Krishnakumar et al., Current Applied Physics 9, 2009, p. 467-471) describes this method for forming a CTO layer at relatively low substrate temperature (500° C.). However, the weight ratio of the tin and cadmium compound precursors dissolved in the spray solvent were not changed during the formation of the CTO film. Therefore, the resulting CTO film has an essentially constant elemental concentration over the thickness of the layer.